The Resonator Fiber Optic Gyroscope (RFOG) has potential of meeting the needs in many areas of the inertial sensing market. To overcome optical backscatter errors, currently available RFOGs lock the clockwise (CW) and counter-clockwise (CCW) laser frequencies onto different longitudinal modes of the gyro sensing resonator. These technologies separate the counter-propagating laser frequencies and up-convert the backscatter errors well above the rotation measurement frequency band. However, if only two lasers are used, the gyro sensing resonator free spectral range (FSR) becomes a part the rotation measurement. Thus, the gyro sensing resonator free spectral range must be measured with great precision to reduce the adverse effects on the sensing of the rotation.
To accurately measure the FSR, currently available RFOGs use a third laser frequency to probe the sensing resonator. It is difficult to modulate three lasers to detect resonance in a way that modulation imperfections do not cause large errors, since one laser is a master laser and the other two lasers are slave lasers. Slave lasers are modulated with high precision, but it is difficult to modulate the master laser with high precision.
Other currently available RFOGs use a master laser and three slave lasers. In this latter technology, the master laser is not used for rotation sensing and the three slave lasers are modulated with high precision. However, the number of lasers and associated phase lock loop electronics results in a significant increase in the cost, size, weight, and electrical power consumption of the RFOG.